Rebel factions that have until now fought Mr Assad's government in separate, largely independent battles are now working together to create one, large 'liberated' area of Damascus from where they could launch a major assault on the city.

The movement represents the most serious challenge to the president's hold on the Syrian capital in months.

"The rebels are trying to connect towns in south Damascus so that the Free Syrian Army can move between them and closer inside," a Damascus activist calling himself Saif Dimaskhy told The Daily Telegraph.

Syrian government jets on Wednesday pounded a belt of towns in the southern suburbs of the capital, from Daraya to Eastern Ghouta in a bid to halt the rebel advance. Videos posted on YouTube recorded the roar of the planes and the plumes of white smoke filling the skies over the district as the bombs hit the ground.

"Daraya has been besieged for 14 days now," said a rebel calling himself Abu Yaman, speaking by Skype from the town. "Cannons and Mig planes are shelling it. The Free Syrian Army have been joined by fighters from Qadam town nearby and they are keeping the regime back on the ground."

Much of the fighting in Damascus in the past months has seen men creating localised rebel groups, with little coordination. In recent weeks the offensive has become more concerted and ever closer to President Bashar al-Assad's seat of power.

On Wednesday, for the first time, a mortar struck the central Damascus district of Abu Rummaneh, a wealthy neighborhood of foreign embassy buildings, expensive restaurants and boutique stores. One person was killed and several others injured, Syria's official media said.

Rebel groups have begun sharing intelligence on enemy positions, an opposition technical engineer calling himself Omar al-Homsi said. He spent weeks listening in to up to 35 different Syrian military radio frequencies.

"They didn't know that we are listening to them so we got a lot of beneficial information such the buildings they were about to target," he said. "I have been setting up a control that shares the information to different brigades." He was speaking from Lebanon where he had come to buy electronic equipment.

Eastern Ghouta, on the outskirts of the capital, has become a center of opposition control. Having seized a regime military base in the area, rebels have acquired anti-aircraft weapons that can bring down government helicopters, activists say. More and more videos are appearing on Youtube showing rebels cheering the smouldering remains of government helicopters.

Rebel cells in the district have formed "weapons" factory, where bombs are manufactured and funneled to other rebel groups, activists have said.

Car bombs inside or close to government military or security centres have become a near daily reality in the capital. "This is the only solution," "Omar al-Homsi" said. "Now we are creating groups to manufacture weapons.

We are making bombs that detonate remotely. It is the only way to destroy the regime's military bases."

Abu Yamen said: "Everyone here is speaking about the final battle for Damascus. I think the regime is losing power on the outskirts of the city.

There are no security forces in Daraya now. They are reinforcing the city centre.

"Security branches and headquarters are recruiting people there and they are sending more tanks to the area."

Separately, Turkey's government requested the deployment of Nato Patriot surface-to-air missiles to bolster its defences along its border with Syria.

Nato's secretary-general, Anders Fogh Rasmussen, said a joint team would visit Turkey next week to conduct a site survey for the US-made Patriots' possible deployment. He also noted that the deployment would not mean imposing a no-fly zone over Syrian territory, a key demand of Syrian opposition groups.